Hi Cheryl,
I can only tell you what works for me.
IF BG is on track before swim, I have a small 15 grm snack before the aswim, to carry me over the swim. If my BG is over 6, I don't tend to need the snack though.
After the swim, test blood. If needed do a small correction, but not until I have driven home. Don't tend to need TBR, if I have eaten after the swim, unless BG is low anyway.
Time of the month does have a big impact though! I also can not be unhooked for more than an hour, without my BG rocketing. It does seem however, that your swim is more intense than mine. So, If I had been doing intense exercise for 45 minutes, I would use a TBR for abut 3 hours, From what you have written, it sounds like to extend teh TBR, or reduce it to stop the crash.